96 Chapter 10 TCHINHADZANDZE GRANODIORITE GNEISS 10.1 Introduction The Tchinhadzandze Granodiorite Gneiss occurs within an area occupied by the Early Proterozoic Messica Granite Gneiss and the Frontier Formation. The distinctive characteristic of this granite is that it does not show the strong planar fabrics which characterize the eastern exposures of the Messica Granite Gneiss and therefore it is either late- or post-tectonic with respect to that deformation. 10.2 Field Description This gneiss occurs as bodies generally associated with the metasediments of the Frontier Formation and their better exposures are at Tchinhadzandze (Fig. 2.2). The granite is grey and contains feldspar which predominates over quartz, biotite and hornblende and locally exhibits feldspar phenocrysts. The rock is weakly foliated, and the foliation is, defined by biotite and hornblende randomly oriented. It is weakly jointed and cut by thin quartz-feldspar and pegmatitic veins. 10.3 Petrography The granite is inequigranular, medium grained (Fig. 10.1 ), and locally exhibits preferred orientation of biotite and hornblende. The feldspars exhibit perthitic and myrmeckitic intergrowths. The mineralogy is dominated by plagioclase and includes quartz, K-feldspar, hornblende and bi.otite. Minor phases are titanite, epidote and zircon (Table 10.1). Table 10.1: Mineralogical assemblage (proportions of each mineral indicated) of Tchinhadzandze Granodiorite Gneiss sample K-fld Pig Qtz Bt Hbl Ep Ttn ygr1 24 42 25 4 4 <1 1 ygr2 23 45 22 4 5 - 1 ygr3 22 45 24 3 5 <1 1 ygr4 23 43 23 5 6 <1 - ygr5 23 42 25 4 5 - 1 ygr6 22 44 24 5 5 <1 - K-fld- potassium feldspar, Pig- plagioclase, Qtz- quartz, Bt- biotite, Hbl- Hornblende, Ep- epidote and Ttn- titanite. K-Feldspar occurs as anhedral grains exhibiting cross-hatched (mostly) and Carlsbad twins whereas plagioclase exhibits albite and Carlsbad twinning. Both feldspar types are cracked and contain inclusions of finer quartz and feldspar grains. Digitised by the University of Pretoria, Library Services, 2012
TCHINHADZANDZE GRANODIORITE GNEISS
Jun 23, 2023
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